Ultrasound waves find several applications in medicine. One such application is ultrasound imaging, wherein ultrasound waves are emitted by an array of ultrasound transducers into the body of a patient and echoes of the ultrasound waves are collected by the ultrasound transducers or by dedicated ultrasound receivers and processed to generate an ultrasound image, e.g. a 1D, 2D or 3D ultrasound image. Another application is ultrasound therapy such as high intensity focused ultrasound (HIFU) therapy in which ultrasound beams are generated by ultrasound transducer elements and are focused on diseased tissue. The significant energy deposition at the focus creates local temperatures in the range of about 65° C. to 85° C., which destroys the deceased tissue by coagulative necrosis.
Such applications face several challenges. For instance, in imaging applications it is far from trivial to achieve a good contact between the ultrasound transducer array and the part of the body to be imaged. This is typically achieved by using special gels that improve the contact between the ultrasound transducer array and the body part. However, a drawback of this approach is that usually large amounts of gel have to be used, which may contain air bubbles that interfere with the transmission or reception of the ultrasound signals. Moreover, the ultrasound transducer array, e.g. in the form of the probe, is typically hand-held during an imaging procedure, which makes the procedure prone to errors.
Similar challenges exist in therapeutic applications, where the focused beam requires periodic readjustment to treat multiple regions of the diseased tissue. This may be done manually by adjusting a focusing element or by beam steering by adjustment of the relative phases of the signals generated by the respective ultrasound transducer elements. The manual adjustment is prone to inaccuracies and the range of phase controlled beam steering may not be sufficient to reach all diseased tissue without array displacement. A further complication is that therapeutic treatments such as HIFU treatments is often monitored by magnetic resonance imaging, such that the materials in the ultrasound transducer array must be compatible with magnetic resonance techniques, e.g. must be diamagnetic.
US 2008/0125661 A1 discloses an ultrasound transducer array including a shape memory alloy. This allows the array to be switched between two geometries, namely an actual geometry and a geometry ‘remembered’ by the shape memory allow. The memory of the alloy may cause a desired change in array geometry during manufacture, such as a shape memory alloy twisting an array into a curved or helix due to applied heat. The array may be fixed in place by the memory alloy or bonding after positioning by the memory alloy, preventing further substantial alteration. It is a drawback that such an array is difficult to control and must be controlled by heat, which is problematic.